It has been reported that individuals in the USA spend up to about 90% of their time indoors. Because poor indoor air quality has been linked to respiratory illnesses, allergies, asthma and sick building syndrome, adequate ventilation and indoor air quality are important for the health, well-being, productivity and thermal comfort of building occupants. However, heat gains and losses through infiltration and ventilation are believed to account for a significant amount of the energy required to maintain comfortable conditions within buildings. Consequently, in an effort to save energy by reducing shell heat gains and losses, the construction of the building envelope has become increasingly tighter. Increased airtightness of buildings results in less ventilation, with the result that the benefits of lower energy requirements are generally obtained at the expense of adequate indoor air quality.
For commercial buildings, indoor air quality can be regulated by air systems that supply air to the indoor space by mixing fresh outdoor air with return air from the indoor space. In residential buildings, however, outdoor air typically enters the space through doors, operable windows, and infiltration. During the heating and cooling seasons, ventilation is usually limited to infiltration because residential air systems typically use only recirculated air and residential hydronic systems heat air through convection with no direct air exchange. The low ventilation associated with these systems can increase indoor pollutant levels because air pollutants (for example, emissions from indoor sources) are not able to escape the home, and insufficient outdoor air is available to dilute indoor air pollutants.
In view of the above, measures for providing adequate fresh air to residential buildings are being explored, with particular emphasis on achieving improvements in indoor air quality with minimal energy usage. In recent years, integrated sustainable design concepts have been adapted that can improve indoor air quality in buildings while conserving energy. For instance, ventilated building facades are currently being integrated into commercial buildings. However, this technology has not been utilized as frequently in residential buildings because of expense and because multistory facades may not be applicable to residential designs. Another approach is windows having a ventilation capability. An example is an airflow window, which as the name implies differs from a conventional window by the existence of internal airflow, in the form of free or forced convection through an airflow cavity between two layers of glass (glazing). The potential for using airflow windows in residential construction has been explored because they are not as complicated as ventilated facades and have the potential for improving indoor air quality and conserving energy for heating and cooling while also allowing daylight to enter a room.
The airflow cavity of an airflow window is usually combined with a double-glazed insulated unit (two layers of glass spaced apart and hermetically sealed with an air space therebetween), resulting in a triple-paned construction. However, various combinations of single panes or double-glazed insulated units can be used to form an airflow window. Four main modes of operation have been reported for airflow windows: supply, exhaust, indoor air curtain, and outdoor air curtain. These modes are respectively represented in FIGS. 1 through 4, which depict outside air being to the left of each window 100, 200, 300, and 400, respectively, and the inside air being to the right of each window. Typically used during the heating season, the supply air window 100 (FIG. 1) draws air from the outdoor space (e.g., outside) 102 to the indoor space (e.g. a room) 104 through an airflow cavity 106 between an outside glass pane 108 (represented as a single pane) and an inside glass pane 110 (represented as a double-glazed insulated unit). Conversely, during the cooling season, the exhaust air window 200 shown in FIG. 2 exhausts air from the indoor space 204 to the outdoor space 202 through an airflow cavity 206 between an outside glass pane 208 (represented as a double-glazed insulated unit) and an inside glass pane 210 (represented as a single pane). FIGS. 3 and 4 show the indoor and outdoor air curtain windows 300 and 400, respectively, as having airflow cavities 306 and 406 that define airflow paths from inside to inside and outside to outside, respectively. In FIG. 3, the airflow cavity 306 is between an outside glass pane 308 (represented as a double-glazed insulated unit) and an inside glass pane 310 (represented as a single pane), and in FIG. 4 the airflow cavity 406 is between an outside glass pane 408 (represented as a single pane) and an inside glass pane 410 (represented as a double-glazed insulated unit). In all cases, airflow is typically from bottom to top as a result of the configurations making use of the thermal buoyancy effects as air increases in temperature. It has been reported that the exhaust air window 200 may also be used during the heating season with airflow from top to bottom.
In general, the working principle of an airflow window is to entrain the solar heat that has been captured by the airflow window and direct the solar energy indoors or outdoors, depending on the operating mode of the window. Captured solar energy is used to preheat outdoor air in the supply mode of FIG. 1, and reheat indoor air in the indoor air curtain mode of FIG. 3. This working principle is ideal for use during the heating season. For the exhaust and outdoor air curtain modes of FIGS. 2 and 4, airflow is used to remove solar energy by convecting away the excess heat during the cooling season and decreasing conductive heat losses during the heating season. The supply air window 100 can also be used for night cooling.
Airflow through the supply airwindow 100 is mainly driven by buoyant effects. Solar energy absorbed by the window 100 heats the air inside the airflow cavity 106. The heated air rises, causing the air in the cavity 106 to stratify and move in an upward direction. The strength of the buoyant forces is governed by the vertical temperature differences in the airflow cavity 106, which is influenced by the height of the window 100. In general, the taller the window 100 and/or the greater the temperature difference, the greater the buoyant force. To ensure airflow into the room when buoyant forces are weak, the supply air mode requires that the room 104 in which the window 100 is located be kept at a slightly negative pressure. Airtight construction in the rest of the room 104 is also essential for achieving airflow only through the window 100.
As compared to a conventional window, the exhaust air window 200 can improve thermal comfort conditions by tempering and then exhausting room air between the two glass panes 208 and 210. This is beneficial during both the heating and cooling seasons because the airflow cavity 206 is respectively warmer or cooler. The decrease in temperature difference between an occupant of the room 204 and the surface of the inside glass pane 210 decreases the radiation exchange and improves thermal comfort. The temperature of the airflow cavity 206 also helps to reduce conduction losses through the window 200. Air can be exhausted by natural effects or mechanical effects by positively pressurizing the room 204.
Although the air curtain modes cannot be used to improve indoor air quality or meet ventilation requirements, they offer benefits related to energy consumption and thermal comfort. The outdoor air curtain 400 of FIG. 4 is most beneficial on a sunny day during the cooling season. Warmer outdoor air is driven upward through the airflow cavity 406 because of buoyancy effects. As the air is heated in the cavity 406, it is drawn to and exhausted from the top of the window 400, which in turn causes air to be drawn from the outdoor space 402 into the airflow cavity 406 through an opening at the bottom of the cavity 406. In this way, the daylighting benefits from solar radiation can be enjoyed without overheating the window 400 and subsequently increasing the temperature of the room 404. By helping to prevent overheating in the airflow cavity 406, the temperature difference between the outdoor space 402 and indoor room 404 is minimized, which reduces heat transfer through the window 400 into the room 404 and consequently decreases the amount of energy needed to cool the room 404.
The indoor air curtain window 300 of FIG. 3 works in a similar fashion during the heating season. Solar energy is absorbed by the air within the airflow cavity 306, causing the air to become heated and rise through the cavity 306, and finally convected to the indoor space/room 304 through an opening at the top of the window 300. The rising air within the cavity 306 causes cooler air to be drawn from the room 304 into the airflow cavity 306 through an opening at the bottom of the cavity 306.
Airflow windows are most effective when installed on the south facade of a building because the increased incident solar radiation on the west and east facades can promote overheating of the window. On the other hand, an airflow window installed on the north facade may not receive enough incident solar radiation during the winter months to effectively temper air supplied to the building. Therefore, for most climates, airflow windows are limited to installation on the south facade.
The airflow window designs described above have several limitations. For instance, only the supply air mode offers the potential for improving indoor air quality by drawing fresh air from an outdoor space 102 into the room 104. Several limitations to the implementation of these airflow windows also arise from the design of their airflow cavities 106, 206, 306, and 406, which are open and as a result raise issues concerning security, acoustics, air quality, cleaning and maintenance, thermal comfort and/or condensation. For some building locations, conventional windows are useful to attenuate outdoor noise, whereas the airflow cavities 106, 206, 306, and 406 of the airflow windows 100, 200, 300, and 400 may provide a channel for outdoor noises to enter the indoor space 104, 204, 304, and 404, potentially causing acoustic problems. The ability to filter outdoor air before it enters a building in the supply air window 100 is important when considering indoor air quality. However, filters can hinder the effectiveness of natural ventilation. Airflow in the airflow cavities 106, 206, 306, and 406 of all airflow window modes can also promote the collection of dirt and dust on the interior surfaces of the window. Though offering the benefit of preheating air that enters a building during the day during the heating season, the supply air window 100 can contribute to heat losses during the night when the temperature of the inner pane 110 can drop, affecting the thermal comfort of the building occupants. Finally, condensation in an airflow window may occur if the surface temperature of a glazing layer falls below the dew point temperature of the air it contacts. Moisture can accumulate at the base of the window, which can lead to damage of the materials used to construct the window. Additionally, high outdoor humidity levels can increase the humidity indoors and decrease thermal comfort.
Other shortcomings of airflow windows are due to their added complexity as compared to a conventional window. The initial cost of purchasing an airflow window is likely higher, though strongly dependent on the type of airflow window and exact construction, as well as the availability of the product in relation to the building location. However, the use of airflow windows may reduce the size of the HVAC system required to heat and cool and building, providing a significant trade-off for the increased cost of an airflow window.
In view of the foregoing, though airflow window technology offers significant potential benefits including improved indoor air quality and reduced heating/cooling loads, current airflow windows have a number of limitations and as such further improvements in their construction and effectiveness would be desirable.